The 1Password key disappeared from my browser toolbar after Firefox updated itself to 29.0.1. I tried updating my 1Password app installation to 3.8.22 (build 32010) then removed and re-installed the browser extension. When Firefox restarted, the key button was there. But on a second browser restart, it was gone again.
This is on Mac OS X 10.9.3.
I saw a similar problem in the Windows forum, but the workaround involved an anti-virus tool that doesn't apply in my environment.